Working fluid charge reduction, part 1: CO2 fin-tube evaporator designed for light commercial appliances utilizing flow pattern based phenomenological prediction models

• Thermal performance of CO2 evaporators.
• Reduction in volume and working fluid charge.
• Two-phase flow pattern and physically-based prediction models for CO2.
• Modelling of tube size and shape, and oil effects on CO2 flow boiling.
• Volume and refrigerant charge reduced by 50% and 58.7%.

The present study focused on improving the thermal performance of CO2 evaporators while reducing their volume and thus significantly reducing the working fluid charge. To achieve this, the leading two-phase flow pattern and physically-based prediction models for flow boiling of CO2 and void fraction were selected from the literature, together with the leading air-side flow and heat transfer methods, and finally those for modelling oil effects on CO2 flow boiling. The effects of tube size, shape and wettability for significant performance improvements, miscible oil concentration, etc., were addressed while staying within the beverage industry's fabrication and operational limits. Compared to an actual existing design, the volume and refrigerant charge of the CO2 evaporator were reduced by 50% and 58.7%, respectively. The adverse influence of oil was found to reduce the mean CO2 side heat transfer coefficient and to increase the CO2 pressure drop by up to 11% and 94%.